FIG. 1 shows a truck tractor or other vehicle 10 having a front axle 12. A driver's side wheel coupled to front axle 12 is indicated at D1 while a passenger side wheel coupled to axle 12 is indicated at P1. Note: The designations D and P are for convenience only as the vehicle may be a left hand drive vehicle or a right hand drive vehicle. Truck or other vehicle 10 has tandem rear axles 14,16. Axle 14 has wheels D2 and P2 coupled thereto. Axle 16 has wheels D3 and P3 coupled thereto. More than one wheel may be coupled to each end of the axles. As is conventional, if two wheels are mounted to an end of an axle, the wheels are typically mounted by the same lug nuts and rotate together. These wheels are thus braked together if a parking brake is located in association with these wheels.
It is common for trucks to include parking brakes. Spring loaded cylinder actuated parking brakes are a specific example. For example, the brakes may be released by filling a cylinder with pressurized air in response to a control signal, such as an electronically delivered or mechanically delivered control signal. In contrast, air may be bled from the cylinder to set the brake. In another common approach, a two compartment brake actuator may be used to brake a wheel at the end of an axle (or plural wheels if more than one wheel is at the end of the axle). For example, the service brake is applied by pushing a brake pedal in the cab of the vehicle. In response to the application of the brake pedal, pressurized air is supplied to one compartment of the actuator resulting in the application of the brake at the wheel. Simultaneously, the other brakes (at other wheels) are applied in this service brake application mode. In this example, the parking brake is applied by operating a parking brake control, such as by moving a lever. In response, air is bled from another compartment of the actuator. This allows a spring to apply the brake of the wheel in a parking brake application mode. Simultaneously, the other brakes (at other wheels) are applied in this parking brake mode. Other forms of parking brake application mechanisms may alternatively be used.
FIGS. 1, 2 and 3 illustrate common known approaches for parking brakes. In FIG. 1, the parking brakes are located at the wheels coupled to the frontmost rear axle 14 (wheels D2 and P2). In FIG. 2, the parking brakes are located at the wheels coupled to the rearmost axle 16. Typically, both axles 14 and 16 are drive axles. As shown in FIG. 3, another known approach involves having parking brakes at wheels (P2 and D2) coupled to the frontmost axle 14 of rear axles 14, 16 together with parking brakes at wheels (P3 and D3) coupled to the rearmost axle 16. The location of the parking brakes is indicated schematically in FIGS. 1–3 by shading in the wheels that have associated parking brakes.
Thus, in the known approaches, if one wheel has a parking brake, a directly opposed wheel on the same axle will have a parking brake. In addition, the parking brakes are operated together. Thus, in FIG. 1, wheels P2 and D2 are both braked when the parking brakes are applied. In FIG. 2, wheels P3 and D3 are both braked when the parking brakes are applied. In addition, in FIG. 3, all four wheels P2, D2, and P3, D3 are braked when the parking brakes are applied.
This approach can lead to some disadvantages.
For example, it is common for vehicles to have air suspension systems for leveling the vehicle. Assume in FIG. 6 that the parking brakes are located only at the rearmost axle 16 (e.g., at wheels P3,D3) in FIG. 6. If the vehicle happens to be parked with the rear wheels at a lower elevation (for example with the rear wheels P3,D3 off the pavement and the front wheels P2,D2 on the pavement) as illustrated in FIG. 6, it is possible for the effectiveness of the parking brakes to be hampered. In this example, the leveling system over time may attempt to raise the rear axle 16 to level the frontmost and rearmost axles 14,16 of the tandem pair. This can result in the rear wheels being lifted off of the ground (if the difference in elevation between the front and rear wheels is great enough) or unloaded enough such that the parking brakes no longer as effectively hold the truck in a parked position. If the truck is on a sufficient incline, movement of the truck is possible.
The present invention relates to all novel and non-obvious features and methods disclosed herein, both alone and in various combinations and sub-combinations with one another, as set forth in the claims below. The invention is not limited to a system which overcomes all of the disadvantages of the prior art. In addition, the invention may be implemented using conventional parking brakes as well as with parking brakes which are developed in the future.